From their web site:
Public.Resource.Org has discovered that the Internal Revenue Service has posted the Social Security Numbers of tens of thousands of Americans on government web sites. The database in question contains the filings of Section 527 political organizations such as campaign committees. This Section 527 database is an essential tool used by journalists, watchdog groups, congressional staffers, and citizens. While the public posting of this database serves a vital public purpose (and this database must be restored as quickly as possible), the failure to remove individual Social Security Numbers is an extraordinarily reckless act.
On July 2, Public.Resource.Org discovered this systematic violation of Americans’ privacy and notified the U.S. Treasury Inspector General for Tax Administration. We documented our findings in an audit document, copies of which were furnished to I.R.S. officials and senior White House officials. On July 3, the administration removed this database from public view.
Public.Resource.Org uncovered this serious violation of federal law in the course of an unrelated audit which was sparked when, on June 18, the I.R.S. notified Public.Resource.Org that it had sent out an improperly-vetted shipment of data on DVD for the January release of the Form 990-T, the Exempt Organization Business Income Tax Return. Because the I.R.S. had publicly released that data in February, and had not notified recipients of the bulk data subscription of this privacy breach for several months, Public.Resource.Org conducted a systematic examination of the breach and how it was handled and delivered that audit to the Inspector General on July 1, 2013.
Read more on PublicResources.org
h/t, Jonathan Ezor